There are already known various constructions of optical waveguides, including optical fibers are provided with embedded gratings that are being used either for inserting light into or for removing light from the respective optical waveguide at an intermediate location or at different intermediate locations of the waveguide. So, for instance, the U.S. Pat. No. 4,749,248 to Aberson, Jr. et al, issued on June 7, 1988, discloses a device for tapping radiation from, or injecting radiation into, a single mode optical fiber. This patent discloses that it is possible to convert a guided mode in an optical fiber into a tunnelling leaky mode or vice versa by forming a grating of appropriate periodicity at least in the core of the optical fiber, and either to remove the guided mode from the fiber core into the cladding by converting it into the leaky mode, and ultimately from the fiber altogether, or to insert light of an appropriate wavelength into the core to form a guided mode therein by directing light of a proper wavelength from the exterior of the fiber toward the grating to propagate in the fiber cladding and to be converted by the grating into the guided mode in the fiber core. It is disclosed in this patent that the grating may be formed mechanically or by exploiting the photoelastic or photorefractive effect; in either case, the grating is formed in such a manner that fiber core regions of identical optical properties are situated in planes oriented normal to the longitudinal axis of the optical fiber.
While this approach may achieve satisfactory results for some applications, it has an important disadvantage in that it results in very high losses of optical power coupled out of or into the optical fiber. This is at least partially attributable to the fact that, inasmuch as the grating is imposed normal to the longitudinal axis of the core, the conversion of the guided mode into the leaky mode or vice versa takes place with uniform distribution all around the fiber axis, so that a predominant proportion of the leaky mode is not captured by the sensing arrangement when this approach is being used to tap light out of the fiber, or bypasses the fiber core when this approach is being used to launch light into the core via the cladding mode and its conversion into the guided core mode at the grating.
It is also already known, for instance from the commonly owned U. S. Pat. No. 4,725,110, issued on Feb. 16, 1988, to impress periodic gratings into the optical fiber core by exposing the core through the cladding to the interference pattern of two coherent ultraviolet light beams that are directed against the optical fiber at two angles relative to the fiber axis that complement each other to 180.degree.. This results in a situation where the grating is oriented normal to the fiber axis so that it reflects, of the light launched into the fiber core for guided propagation therein in a propagation direction, only that having a wavelength within a very narrow range, back along the fiber axis opposite to the original propagation direction so that such reflected light is guided in the core to the point at which the original light had been launched into the fiber core. On the other hand, this grating is transparent to light at wavelengths outside the aforementioned narrow band so that it does not affect the further propagation of such other light. It may be seen that this approach has its limitations as well in that it is not suited for removing meaningful amounts of light from or launching them into the fiber at any other location than the respective fiber ends.
Accordingly, it is a general object of the present invention to avoid the disadvantages of the prior art.
More particular, it is an object of the present invention to provide an optical waveguide with an embedded light redirecting arrangement which does not possess the disadvantages of the known arrangements of this kind.
Still another object of the present invention is to develop the light redirecting arrangements of the type here under consideration in such a manner as to obtain highly efficient coupling of light at a selected wavelength within a limited range between the optical waveguide core and a spatially limited path extending externally of the core.
It is yet another object of the present invention to devise an optical system utilizing the embedded grating optical waveguide of the above type, which system is instrumental in providing for the efficient coupling of light into and out of the optical waveguide.
Yet another object of the present invention is to design the system of the above type in such a manner as to be relatively simple in construction, inexpensive to manufacture, easy to use, and yet reliable in operation.
A concomitant object of the present invention is to develop a method of forming the embedded tap in the optical waveguide core, which method is highly efficient and reliable.